1. Field of the Invention
This invention relates to a fluorescent display apparatus, and more particularly to a composite fluorescent display apparatus of the type which permits the luminous display to be observed through a transparent substrate on which various functional elements of the fluorescent display apparatus are formed.
2. Description of the Prior Arts
A fluorescent display apparatus effects luminous display in the form of letters, figures and the like by selectively impinging electrons emitted from heated cathodes upon anodes having phosphor layers deposited thereon. The fluorescent display apparatus is widely used as a display device in various electronic appliances and instruments, because it can be driven at a relatively low voltage and is low in power consumption and emits luminous color which is easy to observe.
The fluorescent display apparatus generally may be classified in two categories. One involves observing the luminous display of phosphor layers deposited on the upper surface of anodes formed on an insulating substrate through a grid, cathode and front cover, and the other is to observe the luminous display of phosphor layers through a transparent substrate on which there are formed various functional elements of the fluorescent display apparatus, such as, for example, anodes made of transparent conductive films having the phosphor layers deposited thereon, grids and cathodes, all of which are contained in a vacuum envelope covered with a back cover.
In the fluorescent display apparata explained above, the vacuum envelope which is made of the substrate and the front or back cover must be constructed so as to be strong enough to withstand pressure in the case where a large-sized composite fluorescent display apparatus which includes various kinds of display patterns for effecting different types of display in one fluorescent display apparatus is fabricated. In the conventional composite fluorescent display apparatus, such a vacuum envelope is produced by using the substrate and the front or back cover which are made of a thick glass plate or interposing reinforcing members between the substrate and the cover within the envelope. The vacuum envelope produced by these techniques is not totally satisfactory in that the use of the thick substrate and cover increases the overall weight of the fluorescent display apparatus. In addition, complicated processes are required in the production of the cover, which results in an increase in the production costs. Also, problems arise with this vacuum envelope in forming and mounting each of electrodes in the fluorescent display apparatus. On the other hand, the vacuum envelope provided with the reinforcing member is difficult to produce and is not satisfactory from a standpoint of production efficiency. Furthermore, problems arise with the fluorescent display apparatus using the vacuum envelope of this type due to remarkably restriction of the viewing angle of the display in the case where the display is observed from the side of the front cover. In a possible alternating arrangement, there is proposed such a composite fluorescent display apparatus that is composed by assembling a plurality of small-sized fluorescent display tubes on a panel so that the display surfaces of each of the display tubes may be laid on the same plane. However, this composite fluorescent display apparatus is not satisfactory, because it is bulky and less appealing in its appearance, and wirings in each of the display tubes become too congested.